Turbines are widely used in industrial and commercial operations. A typical commercial steam or gas turbine used to generate electrical power includes alternating stages of stationary and rotating airfoils. For example, stationary vanes may be attached to a stationary component such as a casing that surrounds the turbine, and rotating blades may be attached to a rotor located along an axial centerline of the turbine. A compressed working fluid, such as but not limited to steam, combustion gases, or air, flows through the turbine, and the stationary vanes accelerate and direct the compressed working fluid onto the subsequent stage of rotating blades to impart motion to the rotating blades, thus turning the rotor and performing work.
The efficiency of the turbine generally increases with increased temperatures of the compressed working fluid. However, excessive temperatures within the turbine may reduce the longevity of the airfoils in the turbine and thus increase repairs, maintenance, and outages associated with the turbine. As a result, various designs and methods have been developed to provide cooling to the airfoils. For example, a cooling media may be supplied to a cavity inside the airfoil to convectively and/or conductively remove heat from the airfoil. In particular embodiments, the cooling media may flow out of the cavity through cooling passages in the airfoil to provide film cooling over the outer surface of the airfoil.
As temperatures and/or performance standards continue to increase, the materials used for the airfoil become increasingly thin, making reliable manufacture of the airfoil increasingly difficult. For example, the airfoil may be cast from a high alloy metal, and a thermal barrier coating may be applied to the outer surface of the airfoil to enhance thermal protection. A water jet may be used to create cooling passages through the thermal barrier coating and outer surface, but the water jet may cause portions of the thermal barrier coating to chip off. Alternately, the thermal barrier coating may be applied to the outer surface of the airfoil after the cooling passages have been created by the water jet or by an electron discharge machine (EDM), but this requires additional processing to remove any thermal barrier coating covering the newly formed cooling passages.
A laser drill utilizing a focused laser beam may also be used to create the cooling passages through the airfoil with a reduced risk of chipping the thermal barrier coating. The laser drill may utilize a pulsed laser operating at, for example, 5,000 Hz or more. The laser drill, however, may require precise control due to the presence of the cavity within the airfoil. Once the laser drill breaks through a near wall of the airfoil, continued operation of the laser drill by conventional methods may result in damage to an opposite side of the cavity, potentially resulting in a damaged airfoil that must be refurbished or discarded. Accordingly, a sensor, such as a high speed camera having a shutter speed of 110 μs or more, may be used to monitor the laser drill and determine when the laser drill has broken through the near wall of the airfoil. For example, as soon as the camera detects a decrease in an amount of light emanating from where the laser drill is drilling into the airfoil, potentially indicating a breakthrough, conventional laser drills are configured to stop drilling immediately in order to minimize the risk of damaging the airfoil. However, such a method may be inaccurate as the drilling status is determined using a small number of data points. For example, the first frame from the camera that is processed indicating a decrease in the amount of light emanating from where the drill is drilling may trigger the drill to cease operation. Said frame, however, may have been taken between pulses of the laser or otherwise included noise decreasing the accuracy of the measurement. Moreover, such a method requires heavy processing in order to process the large number of frames per second and quickly determine the status of the laser drill.
Accordingly, a method for providing more accurate information regarding the status of the laser drill would be beneficial. In addition, a method of determining the status of the laser drill requiring less processing would be particularly useful.